The present invention relates to a flexible printed circuit board having excellent flame proofness and a very little danger of causing a fire.
In general, flexible printed circuit boards such as the printed circuit of a speedometer for automobiles have heretofore been produced by fastening a metal foil such as a copper foil constituting an electric circuit onto a flexible substrate consisting of a resin film, for example, a polyester resin film with an adhesive. Further, for the insulation of the electric circuit, the metal foil is coated with a flexible film of a resin such as a polyester resin. The flexible printed circuit boards permit the production of a remarkably lighter printed circuit board and a remarkable reduction in cost as compared with printed circuit boards using a phenol resin laminated board as a substrate which have previously often been employed. Therefore, it is expected that the flexible printed circuit boards will have various uses. In the flexible printed circuit boards, however, an inflammable flexible film such as a polyester film is widely used as a substrate and as an insulating film from the viewpoints of cost and processability. Therefore, heat is generated when an excess current flows on a copper foil constituting an electric circuit. In this case, a fuse portion formed in the circuit for safety by narrowing the copper foil burns out and the substrate, the insulating film or the adhesive catches fire. A flame spreads at a fairly high speed and a fire is caused from this portion. Thus, the flexible printed circuit boards are a possible cause of causing a fire. This is the reason why the flexible printed circuit boards have not yet been put to practical use widely.
Therefore, an object of the present invention is to obviate the defect of prior art flexible printed circuit boards.
Another object of the invention is to provide a flexible printed circuit board having excellent flame proofness and a very little danger of causing a fire.